(1) Field of the Invention
This invention relates to an improved polyurethane adhesive composition and more particularly to a dried, partially cured heat-activatable, preformed adhesive film of such an adhesive composition, to its method of manufacture, and its use in the joining together of two adjacent surfaces. More particularly, the invention relates to the splicing together of two free ends of coated abrasive material and, still more particularly, to the splicing of abrasive material as in the formation of endless abrasive belts.
(2) Description of the Prior Art
Coated abrasive belts are manufactured, in general, by adhesively joining together the free ends of strips of coated abrasive sheet material of a suitable length and width. The demands on an adhesive used in the formation of endless abrasive belts are severe, as a coated abrasive belt during use is subjected to relatively high tensile and flexural forces. Thus, any adhesive composition used in the formation of abrasive belt joints must be tough and durable, in addition to providing good adhesion (high peel strength) with the abrasive material, to present delamination of the belt joint upon being subjected to severe flexing conditions, especially when running over a small diameter contact roll. Moreover, the adhesive used in forming the joint must often be resistant to heat, water, lubricants, and other cutting fluids encountered in abrading operations. An adhesive composition found highly suitable for use in the manufacture of endless abrasive belts, and meeting many of the above-disclosed requirements, is a polyurethane composition. Such a composition, in general, involves as the essential components an isocyanate and an isocyanate reactive compound, i.e., a poly-functional active hydrogen containing component.
Polyurethane adhesive compositions, in general, must be used in a relatively short time after formulation and for that reason, among others, have not been overly accepted for use in certain industrial applications. Various means have been suggested whereby a composition of longer shelf or pot life would be provided, one of which is disclosed in U.S. Pat. No. 3,637,359 for Coated Abrasive Belt Overlap Joint. Therein, chlorinated aliphatic and aromatic hydrocarbons and alkylated polystyrene resins are disclosed for accomplishing this purpose. In addition to teaching the incorporating of e.g., chlorinated aliphatic hydrocarbons for longer pot life, it is disclosed that, where pot life is of lesser concern, greater initial joint strength is attained and higher joining temperatures can be used, if a catalyst is also incorporated in the polyurethane composition.
Although liquid adhesive compositions of glue and various resinous and rubbery like materials have long been used in the manufacture of abrasive belts, their use is attendant with certain disadvantages in addition to involving messiness and potential errors in preparation and use from operator to operator. One disadvantage associated with the use of a liquid adhesive composition is that it, in general, requires a "dwell time" for, among other things, the removal of solvent. Thus, an operator must apply the adhesive composition to the surface or surfaces to be joined, wait for a certain period of time to pass in order for the solvent to evaporate and, in some instances, for the adhesive material to cure to some desired point, before joining the ends together for joint formation.
Curing of some adhesive materials to some degree before joining the adhesive coated surfaces together is absolutely necessary in order that an inordinate amount of time is not taken, e.g., for a press curing cycle and, most importantly, so that the adhesive material will have sufficient cohesive strength prior to pressing that it will not be squeezed out during pressing thus resulting in an adhesive starved joint area and therefore a joint with inferior bond strength.
On the other hand, the "open time" or "permissible dwell time", during which the faying surfaces of abrasive material can be adhesively assembled together and a satisfactory bond achieved, must also be observed by the operator. Although it is sometimes possible for an operator to coat other abrasive strips for belt formation during the "dwell time", thus setting up a situation where abrasive belts can be somewhat continuously formed, such a procedure is often confusing to the operator. Often-times abrasive strips adhesively-coated during the "dwell time" for other previously coated strips, for one reason or another, are improperly kept account of. This results in some adhesive compositions not receiving the "required dwell time" and others receiving too much, i.e., exceeding the "permissible dwell time". The result in either instance may be formation of an inferior belt joint.
A further disadvantage associated with the use of a liquid composition results from the fact that different operators, in preparing the coated abrasive ends for belt formation, invariably apply more or less of the adhesive composition than is required for optimum results. The adhesive composition, in some instances, to provide a layer of adhesive material sufficiently thick for provision of a bond suitable strength, must be applied in multiple coats because application of a thicker layer by means of a more viscous composition is not possible.
Because of the nature of a liquid adhesive composition and its manner of preparation and application, the quality of a joint formed therewith often varies somewhat, even from belt-to-belt over a particular period of manufacture. This lack of uniformity in the manufacture of endless coated abrasive belts is naturally accompanied by a lack of uniformity in results in use. This, of course, results in complaints from users of the belts.
Rather than using a liquid adhesive composition in coated abrasive belt manufacture, Applicant has heretofore suggested the use of a preformed adhesive film. Such an adhesive film of a polyurethane composition is disclosed in each of the above-mentioned patent applications from which this application depends. A further improved preformed adhesive film for coated abrasive belt manufacture of somewhat similar composition is disclosed in application Ser. No. 152,694 filed June 14, 1971, now U.S. Pat. No. 3,790,555, by Applicant herein and Matthew T. Gladstone; the patent discloses an adhesive composition which is the partially cured reaction product, in dry film form, of a component having available free isocyanate groups, a first active hydrogen containing component having a hydroxyl number of from about 2 to 15, and a second active hydrogen containing component having an active hydrogen equivalent weight of from about 27 to less than about 500. Other prior art exemplary of the use of preformed adhesive films are U.S. Pat. Nos. 2,060,906; 2,778,169; 2,794,726; 3,406,053; and 3,449,280. As disclosed in these patents, adhesive films have been suggested for use in the bonding together of various lamina. The formation of endless coated abrasive belts is disclosed in, e.g., U.S. Pat. Nos. 2,778,169 and 2,794,726.